Properties of Carbonatite Magma and Processes in Carbonatite Magma Chambers

The physical and thermal properties of carbonatite magmas are estimated by analogy with alkali carbonate melts, which are ionic liquids, composed of discrete, unpolymerized ions. Carbonatite magmas are estimated to have low viscosity (on the order of 5 × 10-2 poise), low heat of fusion (175 J/gm), and large thermal diffusivity (4 × 10-3 J/cm-sec-K) compared to silicate melts. Dissolved water will reduce viscosity; dissolved orthosilicate will have little effect on viscosity because silicate tetrahedra will remain as unpolymerized, discrete anions. Solution of polymerized silicate compounds will increase melt viscosity as long-chain or network silicate polymers form; exsolution of polymerized silicate melt will limit the concentration of long-chain or network silicate polymers in the carbonate melt and provide an upper limit on viscosity. Given estimates of the physical and themal properties of carbonatite magma, it is possible to evaluate the importance of possible physical processes in carbonatite magma chambers. Crystals, even calcite, will settle rapidly through a carbonatite melt; calculated velocities for 1 mm diameter grains range from 2 to 9 cm/sec depending on density. In a carbonatite magma chamber, convection is assumed to be driven by horizontal temperature gradients and will be turbulent (Ra = 1017–1020). Maximum flow velocities in the interior of the intrusion will be too low to suspend crystals and allow them to grow above millimeter size. Growth onto the chamber walls will progress at cm/yr, and allow a significant thickness of carbonatite to grow in situ. Sub-liquidus through subsolidus plastic deformation of such rock could produce the gneissic textures common in carbonatites.